I think the problem lies in the layers of felt on the roof. If they've got under there and have probably been breeding there for years, you will never eradicate them by just treating the interior. If you want to keep the coop, could you strip the roof off, then treat all the edges and joints before re- roofing with Onduline? Or, sadly, it may be necessary to call it a day on this coop, it may be beyond complete recovery. What you are so valiantly trying to do must be getting costly, in time, effort and stress, as well as money and exposure to chemicals.
I understand about the issue with the elderly neighbour, but if he just wants to watch the chickens, does it matter whether they're in the original coop or not?
I understand about the issue with the elderly neighbour, but if he just wants to watch the chickens, does it matter whether they're in the original coop or not?